The present invention generally relates to radio frequency (RF) transmitters, and particularly relates to RF polar modulation transmitters.
The advent of data-driven applications has spurred a paradigm shift from circuit-switched to packet-switched based wireless communication devices. To support burgeoning packet-switched applications, improved data transmission rates and spectrum efficiency have become increasingly important considerations when designing packet-based wireless communication networks and devices.
General Packet Radio Service (GPRS), a subsystem of the Global System for Mobile Communications standard (GSM), introduced packet-switched data into GSM networks. GPRS-compliant components use constant-amplitude modulation techniques such as Gaussian Minimum Shift Keying (GMSK) to provide phase modulated RF signals. A newer standard, known as Enhanced Data rates for GSM Evolution (EDGE), doubled the data-rate capability of GPRS for many wireless communication device functions such as e-mail, video cameras, and MP3 players. To accommodate this increased bandwidth while maximizing spectral efficiency, EDGE-compliant transmission components use non-constant amplitude modulation techniques such as 8-state Phase Shift Keying (8PSK) to provide RF signals having both phase and amplitude modulations.
Accurate phase and amplitude control are important aspects of achieving acceptable transmission signal characteristics in non-constant modulation applications such as EDGE for achieving acceptable performance. As a general principle, constant-envelope amplifiers are not suitable for linear amplification of the amplitude modulations present in non-constant envelope signals. More particularly, spectral performance requirements, such as minimization of Adjacent Channel Power (ACP) and the like, impose high linearity requirements on the circuits used for transmitting such signals.
Of course, transmission circuits biased for linear amplification may be used to amplify signals that include both phase and amplitude modulations. However, linearly biased power amplifiers do not operate as efficiently as those biased for saturated-mode operation. Within the context of this dilemma, “polar modulation” transmitter architectures benefit from the efficiency of saturated-mode power amplifier operation while offering potentially high linearity for amplitude modulation.
Polar transmitters operate separately on phase and amplitude modulation components of a signal. For example, processing logic maps digital data to be transmitted into temporally coordinated phase and amplitude modulation information. The phase modulation information is used to generate a constant-envelope signal for input to a saturated-mode power amplifier, and the amplitude modulation information is used to generate an amplitude-modulated supply signal for the saturated-mode power amplifier. That is, the power amplifier's output signal includes phase modulation information imparted by the phase modulation information in the constant-envelope signal applied to the amplifier's input and (linear) amplitude modulation information imparted by the amplitude-modulation supply signal applied to the amplifier's supply connection. This arrangement allows the power amplifier to produce linear amplitude modulations in its output signal despite being biased for saturated-mode operation.
However, because the amplitude and phase modulated components of a signal are processed by different circuit elements, timing alignment between the phase and amplitude modulation paths must be accurate to maintain signal quality. If the phase and amplitude modulation components are not properly time aligned when combined at the power amplifier, signal quality may become unacceptably degraded.
A path delay offset, or mismatch, between the amplitude and phase modulation paths within the polar modulation transmitter occurs as a result of several variables, such as bandwidth variations between the phase and amplitude modulation paths. The phase modulation path generally includes high-bandwidth radio frequency processing elements such as a phase or frequency locked loop for imparting phase modulations. The amplitude modulation path generally includes low-bandwidth processing elements such as a power regulator or driver for imparting amplitude modulations at the power amplifier output stage of the polar transmitter. Amplitude and phase modulation path delay differences, if not properly accounted for, can cause the amplitude and phase modulation information to arrive at the final output stage of a polar transmitter at different times. Left uncompensated, path delay mismatch may cause the transmitter to fail certain performance metrics such as modulation accuracy, e.g., Error Vector Magnitude (EVM) and signal spectrum spread (i.e., spectral quality). Further compounding the path delay mismatch problem is the tendency for the mismatch to vary in response to several factors such as temperature, component degradation, component operating stability (e.g., frequency stability), power supply variation, etc.